who has 3:9 diff gears in 4spd auto? if you do what disadvantages and advantages are there? i m thinking of putting them in but i want to find out more about them in 4speed autos, thanks

You've already got 3.45's in it so the fairly costly job of changing to 3.9's is pointless and rather stupid. All you're going to do is ruin your fuel economy for a minimal performance gain (and more stress on driveline)

Spend the $$$ on a cam, or better yet start saving for a used BA turbo if you wanna go fast....

not dear $350 for 3.9 fitted, but fair point about drive line and fuel, thanks mate

Me Me Me!
I run 3.9's in an EF Auto.
I have no real issues, it sits at 2200rpm at 100k's, I changed the speedo drive to the black ?23? tooth gear and my speedo reads 10k high. Makes it a bit hard to get a real economy reading but I usually get 480k's (by the odo) per tank of Premium, I drive like granny about 60% of the time and the rest is what I would call "very spirited" driving.
The main prob I have is the Auto shift points are a bit out of whack (due to speedo being out) and it tends to kick back when you are only at mid throttle (ie. half gas in 3rd and it will kick back to second at 5000rpm) but from what I have been told a electronic speedo corrector should fix that.

Apart from that - Go for it. It is one way of showing the dunnydore boys that Falcons aren't slow off the line.

Where are you getting it for those sorts of dollars?

you could sell your 3.45 diff for about that and put the 3.9's in a cheapie..

down in geelong mate,

thanks mate, helped me out alot,

3.9s are great on a 4 speed, i reckon it should have been the xr ratio, and the 3.45's the standard ratio.

whoever said theres almost no difference from a set of 3.45's hasnt tried them. theres more of a ratio gap between 3.45-3.89 than there is between 3.08-3.45.

there is more high speed stress on the stock tailshaft at high speeds, ie 150kmh +, as it is spinning faster. other than, in lower speed situations (ie 99.99% of the time) that there is less stress on the system due to there being less torque required to turn the tailshaft.

I noticed almost no fuel difference going to 3.89's. I used a little more on the highways and a lot less around town. the difference being it also gets driven spiritedly every now and again. the only supporting mods are a set of 4480 extractors, and 2.5 mandrel pipe.

never got it down the strip, as the speedo is uncorrected, and would hit the 180 limit before the line (135kmh actual). but it did outdo a vt clubsport in a serious race to 100kmh by a fraction, and they did it in 6.6 when new, so a 6.6-6.8 sec 0-100 time is feasible.

it helps that 2nd tops out just over 100, so theres only one change, and maximum efficiency in getting to 100.

when gunning it off the line it doesnt dip below 4k rpm on the changes, so you might want/need a better cam that makes a bit more top end poke to suit where you will need it when overtaking/going for broke.

its just the best ratio for 4 speeds. you can overtake in top gear easily, and if space is tight 3rd is just a dream.

do it. do it. do it. for that price you would be nuts not to.

thanks man, i think i will doing this mods now, thanks

what would it be like behind a t5?

Okay im convinced, 3.9's or GTFO haha. So what exactly is involved with swapping the crown wheel and pinion over in say for example, my 3.08 open centre diff and what are the gears taken from? Is any sort of machining required? I have no interest in getting an LSD centre yet, but for now would love to get some better accelleration.

most likely rubbish im thinking.

t5 has a nice short first gear, and they have a shorter top gear meaning they rev higher on the highway. first gear will most likely be so short you wouldnt use it daily, you would probably use second for takeing off. first would be reserved for burnouts only lol.

Ill be able to tell you once i get the t5 conversion finished. Im 99.9% sure i will be changing my 3.89:1 ratio, maybe even back to the 3.08 until i can find a good set of 3.45's or 3.23's. got to wait and see what speed/gear ratio i get with the 3.89's first.



I didnt swap mine but I watched my mate do it. the gears that fit are from r31 skylines. automatics had 3.89's and manuals had 3.7's. the pintara's had the 4.11's. from memory of over a year ago you have to undo the tailshaft from the diffhousing. take off the back brakes, and remove the axel stubbs. this isolates your diff center from all connections. then the watts linkage, and handbrake cable line come off in order to get the diff housing cover off. undo the bottom bolts and crack open a drainage point before taking it all the way off. there are 4 bolts holding some U shaped brackets over the bearings on the sides of the diff center. they come off, and you should be able to remove the center. cant remember exactly how the pinion came out, i was off getting drinks from the fridge when that happened. the pinion from the pintaras will be slightly different and needs to be shimmed up with the propper bearing (not sure on exact details).the crown wheel is an exact fit on the center. there are some pop out seals on the pinion that go off/on in a specific order iirc. the tricky bit is shimming up the center properly as the 3.89 pinion head is a lot smaller than the 3.08 meaning the contect surface has mover slightly to the left when looking from the bum of the car. you need to shim the left side of the center to compensate. having spare shims of varying thicknesses helps when doing this. correct contact and lash are important so I hear instalation is basically reversing the dismantaling process.

its the sort of thing you might want someone experienced to do/help you do.

s**t! So it's not exactly a straight forward job. I can certainly get hold of R31 diff gears no worries, but i think i might leave the installation of them to someone qualified haha.

You will definently need a diff spreader.You will not get the carrier/crown assembly out.

i can tell you straight that you dont "need" a spreader. certainly not on an au (78 series housing).

my mate had no troubles at all getting the center out.

dont know what the e series is like, but we did just fine without a diff spreader.